The field of the invention is βKlotho-dependent GLUT-1 regulation.
Fibroblast growth factor 21 (FGF21) was identified based on cDNA sequence homology to other FGFs (1). Phylogenetic and structural analyses have assigned FGF21 to the FGF19 subfamily, which consists of FGF15 (the mouse ortholog of human FGF19), FGF19, FGF21, and FGF23 (2). The FGF19 subfamily members distinguish themselves from the other 15 FGFs in that they function in an endocrine fashion. FGF23 is secreted primarily from bone and acts on the kidney to inhibit phosphate reabsorption and vitamin D biosynthesis (3-5). FGF15 is expressed by intestinal epithelium and is involved in the negative feedback regulation of bile acid synthesis in the liver (6). FGF21 is expressed predominantly in the liver and has emerged as a metabolic regulator of glucose uptake in adipocytes during a search for novel agents with therapeutic potential to treat diabetes mellitus (7, US Pat Publ No. 20040259780). Administration of recombinant FGF21 lowered blood glucose levels in both obese mice and in diabetic mice. Furthermore, transgenic mice that overexpress FGF21 were hypoglycemic, sensitive to insulin, and resistant to diet-induced obesity (7).
FGF21 can activate FGF receptors (FGFRs) and signaling molecules downstream, including FGFR substrate 2α (FRS2α) and 44/42 MAP kinase (ERK1/2), in adipocytes (7-10). However, efforts to demonstrate a direct interaction between FGFRs and FGF21 have failed. In addition, various cell types of nonadipocyte origin including 3T3-L1 preadipocytes do not respond to FGF21 even though they express multiple FGFR isoforms (7). Furthermore, BaF3 cells that overexpress FGFRs require suprapharmacological doses of FGF21 (200-800 nM) to produce a detectable mitogenic response (8-10).
We and others identified Klotho, a single-pass transmembrane protein, as an essential cofactor for FGF23 to activate FGF23 signaling (11, 12). Klotho was originally identified as a gene mutated in the klotho mouse that exhibited phenotypes resembling human premature-aging syndromes (13). Major phenotypic overlaps were observed between Klotho-deficient mice and Fgf23 knockout mice (14, 15). It was determined that Klotho bound to multiple FGFRs and was necessary for FGF23 to bind FGFRs and activate FGF signaling in various types of cultured cells (11).
βKlotho was isolated based on cDNA sequence homology to Klotho (18). The βKlotho gene encodes a single-pass transmembrane protein that shares 41% amino acid identity with Klotho and in addition to adipose tissue is also expressed in liver and pancreas. Mice deficient in βKlotho have overlapping phenotypes with mice lacking FGF15 or FGFR4 (6, 19, 20). These phenotypes include increased bile acid synthesis and increased expression of two key bile acid syntheses, CYP7A1 and CYP8B1, in the liver.